Liquid transport through channels or structures on a micro scale has important implications in a number of different technologies.
Controlled transport of fluids through micro channels has been a challenge, with the microstructure itself imparting difficulties not found on a larger scale. The driving force utilized in most micro channel structures depends on electro-endosmosis, gravitational forces, external pressure or capillary migration.
Surface materials often have unbound electrons, polar moieties, or other features generating a surface charge or reactivity. Surface characteristics often have a more pronounced impact on a micro scale system than on a larger structure. This is particularly true in micro systems where the fluid flow is driven by attractions between liquids and the surface materials through which they are transported.
In a closed capillary the driving force is usually represented by the equation:h=2σgl cos(θc)/gρ  (1)where h=the height of a fluid within a capillary tube; θc=the contact angle of the fluid with the capillary tube material.
If the contact angle of the capillary tube material, with respect to the fluid, is less than 90°, the material is considered hydrophilic. If the contact angle of the tube material, with respect to the fluid, is greater than 90°, the material is considered hydrophobic. σgl represents the surface tension of the fluid with respect to the air (millijoules/m2), g is the gravitational constant (m/s2), r is the radius of the capillary tube (m), and ρ is the fluid density (kg/n3).
Planar micro structures have been developed in which a number of grooves or channels are made, typically such a planar structure is produced by etching grooves in a semiconductor substrate, such as a silicon wafer, and then covering the etched surface by a cover plate to complete the channels. Such structures are, however, rather time consuming and expensive to produce.
Further, when such structures need to be customized, e.g. by the addition of chemical reagents, the functionalization of surfaces etc., these steps often need to be performed by somebody other than the producer of the micro structure. In practice, the micro structures are manufactured at one location and shipped to another facility e.g. for the addition of reagents, whereupon they often need to be returned to the manufacturer for closing and sealing. One aim of the present invention is therefore to make available a micro structure offering greater flexibility and ease, in particular with regard to the post-production customization.
Systems used at present utilize external means, e.g. gravity, centrifugal force (spinning of disk elements with micro channels on the surface), or pressure to impose transport of liquids in channels. Also electric fields can be used to impose transport of dissolved charged species in micro systems. To this end, external auxiliary equipment is employed, such as a motor to generate the spinning of a disk, pumps to create pressure, electrodes and power supplies to apply electric fields etc. Such equipment is costly and sometimes rather complex. Furthermore, in certain cases the forces involved in the above mentioned methods could have detrimental effects on sensitive substances. Another aim of the invention is therefore to make available a micro structure with built-in functionality, removing or reducing the need of external means to impose liquid transport.